1. Field of the Invention
The present invention relates to a brake system with a brake booster, and more particularly to a counter force mechanism of a brake booster.
2. Discussion of the Prior Art
Generally, the brake system in use with a motor vehicle is made up of a brake booster with an input shaft interlocked with a brake pedal, a master cylinder interlocked with the output shaft of the brake booster, and a wheel cylinder operating by the hydraulic output of the master cylinder.
The brake booster comes in two varieties; a pneumatic pressure booster and a hydraulic pressure booster. The pneumatic pressure booster is composed of a valve body slidably disposed within a shell, a power piston provided in the valve body, a fixed pressure chamber and a variable pressure chamber disposed on both sides of the power piston, a valve mechanism for selectively connecting fluid passages located in the valve body, a valve plunger slidable disposed within the valve body and forming in part the valve mechanism, an input shaft for moving forward and backward the valve plunger to cause the valve mechanism to operate for passage switching, and an output shaft being moved forward when the power piston is moved forward.
The pneumatic pressure booster further includes a counter force mechanism. The counter force mechanism has a reaction disc, made of rubber, disposed between the output shaft and the valve plunger. When the booster is operated, the valve body and the valve plunger are simultaneously brought into contact with the reaction disc. A part of brake counter force applied to the output shaft is transmitted to the valve body, and the remaining one is transmitted to the valve plunger. The brake counter force acting on the valve plunger is transmitted to the driver via the input shaft and the brake pedal.
A servo ratio of the brake booster may be varied by varying a ratio of the counter force received by the valve body and that by the valve plunger, viz., a ratio of the pressure receiving areas of the valve body and the valve plunger.
The hydraulic pressure booster is composed of a power piston slidably disposed within a housing, a power chamber formed at one end of the power piston within the housing, a valve mechanism, provided in the power piston, for selecting connecting fluid passages in linking with the input shaft, a fluid supplying passage for communicating the power chamber with a hydraulic pressure source through the action of the valve mechanism, a discharge passage for communicating the power chamber with a reservoir through the action of the valve mechanism, and an output shaft being moved forward when the power piston is moved forward.
In the hydraulic pressure booster, a hydraulic pressure is transmitted from the power chamber to the output shaft, and then transmitted as a brake counter force to the driver.
In the conventional brake booster, the servo ratio is set at a high value so that a large brake hydraulic pressure is produced by a small pedaling force to the brake pedal. In a rapid operation for brake, the brake booster fails to follow the quick braking operation, and cannot produce a large output as expected. This fact implies: that the rapid braking operation is difficult for the infirm driver, e.g., old man and woman.
In the case of the pneumatic pressure booster, upon depression of the brake pedal, the valve mechanism is switched through the action of the input shaft; a pressurized fluid is introduced into the variable pressure chamber; and the power piston and the valve body are moved forward. With the advancement of the valve body, the output shaft is advanced through the action of the reaction disc; the advancement of the output shaft generates a hydraulic pressure; a reaction or counter force to the hydraulic pressure is applied to the output shaft; and the brake counter force applied to the output shaft is distributed to the valve body and the valve plunger.
In the rapid operation for brake, before the power piston and the valve body are advanced by the pressurized fluid introduced into the variable pressure chamber, the valve plunger that is interlocked with the brake pedal through the input shaft is advanced. Most of the brake counter force applied to the output shaft is transmitted to the valve plunger. The result is that the brake counter force transmitted to the driver is considerably large.
The driver must depress the brake pedal overcoming such a large brake counter force. A pedaling force required for the rapid braking is much larger than that in a normal braking operation in which the brake pedal is gradually depressed to produce a large braking force.
This is true for the hydraulic pressure booster.
Accordingly, an object of the present invention is to provide a brake system which is capable of produce a large brake output by a small pedaling force when the brake booster is rapidly operated.
A brake system constructed according to the present invention includes 1) a hydraulic-pressure counter-force mechanism for transmitting a counter force to the input shaft when the brake booster is operated, and 2) counter-force reducing means for reducing the counter force transmitted to the input shaft when the brake booster is rapidly operated.
In the brake system thus constructed, in a normal operation for brake, the counter force is transmitted from the hydraulic-pressure counter-force mechanism to the input shaft. Therefore, the brake system can be operated in a normal manner.
In a rapid operation for brake, the counter-force reducing means reduced the counter force transmitted to the input shaft. Therefore, the brake booster can be operated by a smaller pedaling force in the rapid operation than in the normal operation.
Also, a brake booster according to the invention is structured so that a counter force, which is applied to the input shaft when the brake booster is operated, is not transmitted to the valve plunger, and includes a counter-force piston slidably disposed on the valve body, a buffering member disposed between the counter-force piston and the valve plunger, and pseudo counter force applying means for transmitting an urging force produced by a difference in pressure between the variable-pressure chamber and the constant-pressure chamber, which is exerted on the counter-force piston, to the valve plunger through the buffering member as a pseudo counter force.
With such a construction, when the brake booster is operated, the counter force applied to the output shaft is not applied to the valve plunger. Even when, in a rapid operation for brake, the valve plunger interlocked through the input shaft is greatly advanced before the power piston and the valve body are moved forward by the pressurized fluid introduced into the variable pressure chamber, the counter force applied to the output shaft is not transmitted to the driver via the valve plunger and the input shaft.
The pseudo counter force applying means includes a counter force piston for receiving a differential pressure between the second constant-pressure chamber into which a pressure is introduced from the constant pressure chamber and the second variable-pressure chamber into which a pressure is introduced from the variable pressure chamber. In a normal operation for brake, a pseudo counter force, which depends on a differential pressure between the variable pressure chamber and the constant pressure chamber, is applied to the valve plunger. At the instant that a rapid operation is performed for brake, a pressure in the variable pressure chamber does not increase because of a slow response of the brake booster. Therefore, a pressure in the second variable-pressure chamber also does not increase. As a consequence, a large differential pressure is not applied to the counter force piston. This implies that a large brake output is produced by a small braking force.
The counter force piston is brought into contact with the valve plunger in a state that the buffering member is inserted therebetween. There is no chance of encountering such an unwanted situation that at the instant that the counter force piston hits the valve plunger, the driver feels as if the input shaft is instantaneously forced back. Therefore, a good operation feeling is secured.